PWM controller optimized for use in a synchronous forward converter with active clamp reset.

The LT3753 operates over an input voltage range of 8.5V to 100V, delivers up to 95% efficiency and is targeted for power levels up to 400W. A programmable volt-second clamp provides a safeguard for transformer reset that prevents saturation and protects the MOSFET. This function allows for transformer and MOSFET optimization, resulting in a reduced solution size.

The device output voltage regulates to ±5% without the use of an optocoupler. When an optocoupler is used, ±1.5% regulation can be realized. The LT3753 sends a control signal via a pulse transformer to a secondary-side MOSFET driver for the synchronous rectification timing. It can also be used in self-driven applications (when operating over a narrow input voltage range) where the secondary-side MOSFETs are controlled by the power transformer pulses.

Furthermore, the LT3753 can be used in nonsynchronous designs, typically required for higher output voltage applications.

When the volt-second clamp is set above the natural duty cycle of the converter, it provides a duty cycle guardrail to limit primary switch reset voltage and prevent transformer saturation during load transients. The volt-second clamp also limits maximum VOUT if the opto path breaks open or, alternatively, defines VOUT in no-opto applications.

For non-isolated high step-down ratio applications, the LT3753 contains a voltage error amplifier, allowing a very simple non-isolated fully regulated synchronous forward converter. Additional features include programmable overcurrent protection, adjustable input undervoltage and overvoltage lockout along with built-in thermal shutdown.

The LT3753 has a programmable 100kHz to 500kHz operating switching frequency and can be synchronized to an external clock, allowing the use of a wide range of output inductor values and transformer sizes.

Features

－ Input Voltage Range: 8.5V to 100V

－ Programmable Volt-Second Clamp

－ High Efficiency Control: Active Clamp, Synchronous Rectification, Programmable Delays

－ Short-Circuit (Hiccup Mode) Overcurrent Protection

－ Programmable Soft-Start/Stop

－ Programmable OVLO and UVLO with Hysteresis

－ Programmable Frequency (100kHz to 500kHz)

－ Synchronizable to an External Clock